The term gyrator was introduced by Tellegen to designate the concept of a circuit element embodying the essence of nonreciprocity:
1. B. D. H. Tellegen: "The Gyrator, a New Electric Network Element"; Philips Res. Rep. 3, 81-101 (1948). PA1 2. C. L. Hogan: "The Microwave Gyrator"; The Bell System Technical Journal, Vol. XXXI, No. 1, 1-31 (January 1952). PA1 3. F. Bloch, W. W. Hansen & M. Packard: "The Nuclear Induction Experiment"; Phys. Rev. 70 474 (1946). PA1 4. N. F. Ramsey, Nuclear Moments; Wiley, 1953. PA1 5. C. L. Hogan, "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications" Rev. Mod. Phys., Vol 25, pg 253 (1953). PA1 6. B. Lax & K. J. Button, Microwave Ferrites and Ferrimagnetics; McGraw-Hill, Sec. 12-1, p. 544 (1962).
Thus, where every reciprocal linear circuit device can be represented by an appropriate combination of the four basic element types, inductor, capacitor, resistor, and ideal transformer, Tellegen envisioned that by augmenting these with a fifth element type, the gyrator, every non-reciprocal linear device could be represented as well.
The gyrator is a non-dissipative two-terminal device having forward and reverse transfer phases which differ by 180.degree..
This property might seem to violate the reciprocity principle, a consequence of the symmetry properties with respect to time of the fundamental laws of electromagnetism as expressed by Maxwell's equations, which state that if a voltage is introduced at a first location in a network, and a current is measured at a second location, then the voltage/current ratio will be the same if the locations of the voltage source and current sensor are interchanged.
In fact, the gyrator represents no violation of this general principle at all, but is instead a manifestation, under appropriate conditions, of the distinctive constitutive properties of certain media of electromagnetic propagation, called gyrotropic, which are capable of undergoing a change in their influence on propagating electromagnetic waves under reversal of their state of magnetization. Magnetized ferrites and related magnetic oxides such as YIG (yttrium-iron garnet), magnetoplumbites, and gaseous and solid-state plasmas are examples of gyrotropic materials. Thus, the conventional term "nonreciprocal" should not be taken literally, but only as a convenient designation for this class of phenomena. This property is exhibited with particular clarity in the phenomenon of magnetic resonance induction:
and in microwave Faraday rotation of the polarization:
The Bloch and Ramsey references 3,4 !illustrate magnetic resonance induction at radio frequencies. In these examples, a sphere or other small specimen of gyrotropic material is placed at the center of two mutually orthogonal concentric wire loops or coils mounted in an electromagnet. When excited by an applied radio-frequency signal in one of the coils, the magnetic moment of the specimen is set into precessional motion, inducing an output signal in the other coil. The sense of precession, clockwise or counterclockwise, in response to an oscillating signal is related to the direction of the static magnetizing field. If the static field is reversed, the direction of magnetization and the sense of precession reverse. This, in turn, reverses the phase of the electromagnetic coupling. Likewise, if the roles of input and output connections are interchanged, the direction of the magnetic field being left unchanged, the phase relation between the incident and output signal is reversed. This is the physical basis for the gyrator action.
The principle is the same in the case of the Hogan and Lax & Button references 5,6!, which illustrate microwave Faraday rotation. In these examples, a rod of gyrotropic material is mounted on the axis of a circular-cylindrical waveguide and magnetized axially. An incident linearly polarized microwave signal undergoes rotation of its plane of polarization due to interaction with the precessional magnetic motion which the signal induces in the rod. Here again, the sense of polarization rotation is determined by that of the precession which, in turn, depends on the direction of magnetization. To demonstrate the performance of the microwave gyrator, single-mode waveguides connected at the input and output ends of the cylindrical guide are oriented about their axes as polarizer and analyzer to accept polarizations at 90.degree. relative to one another. Similarly, conditions in the rotator section, principally the diameter and composition of the rod, and magnitude of the applied static magnetic field, are arranged to produce a Faraday rotation angle of 90.degree.. Thus, a signal incident from either end undergoes Faraday rotation so as to be suitably polarized for transmission at the other end with only incidental scattering, but the phase of the transmission is opposite (differing by 180.degree.) in the case of the two directions. Likewise, the phase changes for transmission in the two directions are interchanged if the direction of magnetization is reversed. These are the essential characteristics of the gyrator.
The gyrator has long been a focal point of interest in relation to microwave device and system technology, for its practical utility as well as for its theoretical significance. Gyrators have served as the basic nonreciprocal element in circulators and isolators, which are indispensable in microwave systems of all kinds as means to divide, combine and direct signals and to suppress unwanted reflections in microwave systems. Actual physical embodiments of the gyrator can perform as the non-reciprocal element in many magnetic microwave devices; in addition, the gyrator also serves as a powerful abstract concept for logical representation and analysis in microwave circuit theory. Gyrators are also incorporated into devices which provide other essential circuit functions, such as magnetically controlled switching and phase shifting operations.
Prior Art FIG. 1 illustrates a gyrator employed in a four-port circulator which can function as a signal director, divider or combiner or as an isolator or switch. The circuit is a bridge configuration consisting of two "hybrid" or "magic" T junctions, T1 and T2, connected by two parallel lengths of transmission line 22A, 22B. Each junction T1, T2 includes an even symmetry port (circulator ports 1 and 2) and an odd symmetry port (circulator ports 3 and 4). Consider first an incident signal entering the circulator via port 1. The incident signal 20 is divided into two parts 20A, 20B which are conducted on two lines, or arms, of the bridge 22A, 22B. In junction T2, the two signals 20A, 20B are recombined into signal 24. A gyrator G is positioned in the first arm 22A. The lengths of the two arms 22A, 22B are designed such that, for an incident signal entering port 1, the even-symmetry port of T1, and propagating in the direction from T1 to T2, the two signals 20A, 20B remain in phase and are combined to emerge 24 at the even-symmetry port of T2 (circulator port 2).
Consider now a second signal 26 incident on port 2 traversing through the circuit in the opposite direction from junction T2 to T1. Since the gyrator G furnishes a 180.degree. difference in phase for this signal compared with the first direction of propagation, it follows that, for a signal 26 incident at port 2 and divided with equal phase into signals 26A, 26B, the two signals 26A, 26B arrive at T1 precisely out of phase and are combined into signal 28 at the odd-symmetry port (circulator port 3) of T1 . This illustrates the essential circulator action. Continuing the same logic, a signal 30 incident at port 3 of T1 is divided with an initial 180.degree. phase difference at junction T1 due to the odd symmetry at that port and emerges at the odd-symmetry port of T2 (circulator port 4) as signal 32. Likewise, a signal 34 incident on port 4 and divided with an initial 180.degree. phase difference at T2 is affected by a further 180.degree. phase change due to the gyrator G and emerges at the even-symmetry port of T1 (circulator port 1) as signal 36.
The device described in the above example can be adapted for use as an isolator by designating three ports of each T-junction for the signal path, with the remaining port of each T-junction terminated with matched attenuators. This circuit can also serve as a switch or reversible circulator, by taking advantage of the magnetic-control feature intrinsic to the gyrator: if the direction of magnetization of the gyrotropic element is reversed, the gyrator action is reversed and the 180.degree. phase difference between the two arms of the bridge occurs for propagation in the opposite direction, from T1 and T2, thereby reversing the circulation port sequence outlined above, from 1-2-3-4-1 . . . to 1-4-3-2-1 . . . .
Modern implementations of the gyrator generally require a complicated structure, including a magnetic yoke external to the microwave path, which makes them comparatively large in size and weight and expensive to manufacture. For these reasons, modern gyrators do not lend themselves well to the evolving technology of microwave planar circuits, where minimization of size, weight, and cost are essential.